The present invention relates to direction-finding interferometers, and, more particularly, to autocalibration systems for such interferometers. The invention has application to both phase and gain autocalibration.
Direction-finding interferometers determine direction by ascertaining phase differences in signals received by multiple antennas. In such a system, signals from several antennas are connected to a central location by signal channels, including active components and transmission lines. Any phase differences introduced by signal channels must be precisely compensated or accounted for to maintain system accuracy. In interferometers employing sum signals, used for example to improve signal-to-noise ratios, gain differences also should be compensated to optimize performance of the detection and signal processing circuitry following the formation of the sum signal.
Temporal variations in electrical length (i.e., phase shift) may differ between signal channels due to varying temperature gradients, different reactions to temperature and other environmental factors on the respective signal channels, and equipment aging. These variations are aggravated in long-baseline interferometers in which the antenna elements are spaced far apart so that environmental variations between paths are greater and electrical lengths are more difficult to control.
More specifically, many interferometers employ radio-frequency (RF) preamplifiers between the respective antenna feeds and the central processing location. Generally, the temperatures of these preamplifiers are not closely controlled. Also, the components of the preamplifiers are subject to drift and aging.
Additionally, the signals in the varying paths are usually translated by mixers, amplified by subsequent independent microwave and intermediate frequency (IF) amplifiers, and switched prior to phase and/or gain comparison. All these processes are potential sources of differential errors which can have deleterious effects on interferometer accuracy if left uncompensated.
In order to compensate precisely for variations in electrical length, calibration is necessary. Occasional recalibration is required to correct for temporal variations. An external beacon of known direction may be used for periodic recalibration of the direction-finding interferometer.
For external calibration to be effective, the beacon source must be "in view" and in a known position and orientation relative to the interferometer. Thus external calibration requires independent position location and orientation determination. Furthermore, external calibration can only occur when the interferometer is in the vicinity of a calibration beacon, and there is no significant interference with the calibration beacon. As a result, external calibration is not very suitable for frequent calibration. Hence, there is relatively great opportunity for systematic errors in direction-finding interferometers to develop between external calibrations.
Various internal calibration systems have been developed. Generally, these are used in conjunction with occasional external recalibration so that phase and gain differences introduced by factors, such as thermal distortion of the antenna, other than electrical length may be compensated. However, most internal calibration systems are dependent on known relative electrical lengths, and so are subject to the same temporal errors as the interferometer system.
M. Mollet et al., "Advanced VHF Interferometer Spacecraft Tracking System," Electrical Communication, Vol. 49, No. 3, 1974, discloses a system for reducing errors due to the uncertainty of the electrical lengths of the transmission lines. Bi-directional transmission of two calibration signals at different frequencies through the system is used to determine the electrical lengths of the transmission lines. However, precise calibration is impaired by indeterminable frequency-related phase shifts through the different network paths.
What is needed is an improved autocalibration subsystem for a direction-finding interferometer, which provides for frequent recalibration without requiring bi-directional transmission of calibration signals at different frequencies. The autocalibration should be effective despite different temporal variations in the electrical lengths of the various signal channels. Furthermore, compatibility with external calibration systems is desired.